Introduction to Verilog

Multiplexers

Introduction to Verilog

• Verilog Hardware Description Language (Verilog HDL) released by Gateway Design Automation in 1983

• Cadence buys Gateway in 1989

• Cadence releases Verilog HDL to the public domain in 1990

• Verilog adopted by the IEEE as IEEE standard 1364 in 1995

Combinational Circuit Example

n-line 2-to-1 Multiplexer

n-line

2 x 1 MUX

a(n-1:0)

b(n-1:0)y(n-1:0)

sel

sel y

0 a

1 b

An n-line 2 x 1 MUX

a(n-1:0)

b(n-1:0)

y(n-1:0)

sel

n-line2 x 1MUX

module mux2p(a,b,sel,y);

parameter width = 2;

input [width-1:0] a;

input [width-1:0] b;

input sel;

output [width-1:0] y;

reg [width-1:0] y;

always @(sel, a, b)

begin

if(sel == 0)

y = a;

else

y = b;

end

endmodule

parameter statement defines width of bus

module mux2p(a,b,sel,y);

parameter width = 2;

input [width-1:0] a;

input [width-1:0] b;

input sel;

output [width-1:0] y;

reg [width-1:0] y;

always @(sel, a, b)

begin

if(sel == 0)

y = a;

else

y = b;

end

endmodule

module name

always sensitivity list

Sequential statements (if…else) must be in

an always block

Note optional begin…end

in always block

An n-line 2 x 1 MUX

reg defines y as a variable

module mux2p(a,b,sel,y);

parameter width = 2;

input [width-1:0] a;

input [width-1:0] b;

input sel;

output [width-1:0] y;

reg [width-1:0] y;

always @(sel, a, b)

begin

if(sel == 0)

y = a;

else

y = b;

end

endmodule

Note: = is a blocking assignment

An n-line 2 x 1 MUX

Note: == is

logical equality operator

Digilab2 – DIO1 Boards

Spartan IIFPGA

8 LEDsLD

8 SwitchesSW4 Pushbuttons

BTN

Four 7-segmentdisplays

Pushbuttonbn

74HC373 latch ldg <= ‘1’

Top-level Design – Lab 1

a(3:0)

b(3:0)

mux2g

Lab1

sel

ySW(1:4)

SW(5:8)

LD(1:4)

BTN4

ldg‘1’

a(3:0)

b(3:0)

mux2g

Lab1

sel

ySW(1:4)

SW(5:8)

LD(1:4)

BTN4

ldg‘1’

module Lab1v(SW,BTN4,ldg,LD);

input [1:8] SW;

input BTN4;

output ldg;

output [1:4] LD;

mux2p SWmux(.a(SW[1:4]),.b(SW[5:8]),.sel(BTN4),.y(LD));

defparam SWmux.width = 4;

assign ldg = 1; // enable 74HC373 latch

endmodule

module Lab1v(SW,BTN4,ldg,LD);

input [1:8] SW;

input BTN4;

output ldg;

output [1:4] LD;

mux2p SWmux(.a(SW[1:4]),.b(SW[5:8]),.sel(BTN4),.y(LD));

defparam SWmux.width = 4;

assign ldg = 1; // enable 74HC373 latch

endmodule

a(3:0)

b(3:0)

mux2g

Lab1

sel

ySW(1:4)

SW(5:8)

LD(1:4)

BTN4

ldg‘1’

module Lab1v(SW,BTN4,ldg,LD);

input [1:8] SW;

input BTN4;

output ldg;

output [1:4] LD;

mux2p SWmux(.a(SW[1:4]),.b(SW[5:8]),.sel(BTN4),.y(LD));

defparam SWmux.width = 4;

assign ldg = 1; // enable 74HC373 latch

endmodule

a(3:0)

b(3:0)

mux2g

Lab1

sel

ySW(1:4)

SW(5:8)

LD(1:4)

BTN4

ldg‘1’

Note: assign statement used for

concurrent logic equations

Note: defparam statement

used to override parameter width

An n-line 4 x 1 multiplexer

a(n-1:0)

b(n-1 :0)y(n-1 :0)

sel(1:0)

n-line4 x 1MUXc(n-1 :0)

d(n-1 :0)

Sel y

“00” a

“01” b

“10” c

“11” d

An n-line 4 x 1 multiplexer module mux4p(a,b,c,d,sel,y);

parameter width = 4;

input [width-1:0] a,b,c,d;

input [1:0] sel;

output [width-1:0] y;

reg [width-1:0] y;

always @(sel, a, b, c, d)

case(sel)

0: y = a;

1: y = b;

2: y = c;

3: y = d;

default: y = a;

endcase

endmodule

Must include ALL posibilities

in case statement

Signals can have values0, 1, z, x

sel y

“00” a

“01” b

“10” c

“11” d

Note: default

is decimal

module Lab1bv(SW,BTN,ldg,LD);

input [1:8] SW;

input [1:2] BTN;

output ldg;

output [1:2] LD;

mux4p SWmux(.a(SW[1:2]),.b(SW[3:4]),

.c(SW[5:6]),.d(SW[7:8]),.sel(BTN),.y(LD));

defparam SWmux.width = 2;

assign ldg = 1; // enable 74HC373 latch

endmodule

Top-level design

Verilog Always Block

always @(<sensitivity list>)begin <sequential statements>end

Within an always block:

Variables (reg) are assigned using = (Blocking)

and are updated immediately. (Used for combinational signals)

Sequential signals are assigned using <= (Non-blocking)

and are updated at the end of the always block.